1. Field of the Invention
The present invention relates, in general, to an oscillator device.
2. Description of Related Art
Many microelectronic circuits require the input of some kind of clock signal for the timing of their operation. Generally, such a clock signal is provided by an oscillator device implemented on the same substrate as the microelectronic circuit. The oscillator device is arranged to provide a periodic signal with a substantially constant frequency.
Such integrated circuit oscillators require some kind of frequency reference, which is used to keep the frequency of oscillation constant. Such a frequency reference is usually realized by using accurate external components such as resistors, capacitors and crystal or ceramic resonators.
However, frequency references realized with accurate external components increase the size and cost of an integrated circuit (IC) oscillator. Unfortunately, the components available in a standard IC process are quite inaccurate. Their use means that the frequency of the oscillator's output must be trimmed, which increases manufacturing cost. Alternatively, realizing more accurate components in an IC process results in significantly increased complexity and which also increases manufacturing costs.
From the prior art, thermal oscillators on semiconductor substrates are known in which the generated frequency is determined by the diffusion rate of heat in the substrate. U.S. Pat. No. 6,121,848 discloses a thermal oscillator circuit, which consists of a thermal RC network (i.e. heater and temperature sensor implemented in the same substrate and located some fixed distance apart), and signal processing means, which consists of a limiting amplifier (i.e. a comparator), a multiplier, a low pass filter and a voltage controlled oscillator. During use, the voltage controlled oscillator provides heat pulses to the thermal RC network. The limiting amplifier receives an input signal from the thermal RC network which input signal relates to the temporal temperature obtained at the output of the thermal RC network. The limiting amplifier amplifies the signal with high gain in such a way that the signal is transformed into a square-wave, which reflects the polarity of the input signal.
The oscillator circuit as disclosed in U.S. Pat. No. 6,121,848, when integrated on a semiconductor substrate, displays some undesirable features for the following reasons:                The high thermal conductivity of the substrate requires a relatively large input power to ensure a measurable signal at the output of the thermal RC network (in comparison with the thermal noise of the temperature sensor and the amplifier).        Furthermore, for smaller signal levels the oscillator circuit from the prior art will suffer from inaccuracies inherent to the function of the limiting amplifier. The polarity of an input signal near the threshold level of the limiting amplifier may be randomly altered by thermal noise, causing random fluctuations in the polarity of the amplifier's square-wave output. In this way, small noise fluctuations give rise to much larger changes at the output of the limiting amplifier. The position of an edge of the square-wave signal generated by the limiting amplifier will thus show a variation (jitter) relating to the noise in the input signal from the thermal RC network. This jitter adversely affects the constancy and accuracy of the frequency of the arrangement as a whole. An example of the effect of thermal noise on the function of an oscillator circuit from the prior art is illustrated by V. Székely et al., in “Test structure for thermal monitoring”, Proc. 1996 IEEE Int. Conf. on Microelectronic Test Structures, Vol. 9, March 1996, pp. 111-115 (FIG. 8).        
Due to the aforementioned reasons, the power dissipation of the oscillator circuit from the prior art is relatively high, while its accuracy will be relatively low.